The present invention relates to means for fastening two members together and more particularly to such fastening means having provision for causing a plurality of fastening means to create equal compression forces on the two members being held together thereby.
There are numerous instances wherein two members having mating surfaces must be releasably held together in such a manner that the compressive forces holding the members together are substantially equal over the entire mating surfaces thereof. One well-known example of such an application is the mounting of cylinder heads on reciprocating piston engines and compressors. In such apparatus it is sufficient to have threaded studs passing through bores within the cylinder head to be fastened down by nuts equally tensioned by measured forces such as available with a torque wrench.
Underwater drilling apparatus poses a similar problem but one increased in scale. In such sub-sea drilling apparatus, a riser pipe extends from the ocean floor to a floating platform or vessel on the ocean's surface. Because of the extreme depths involved, the riser cannot be made in one single section. Therefore, the riser comprises a plurality of conduit sections coupled together. Contrary to the cylinder head of the foregoing example which usually remains fastened together for months or years on end, the marine riser conduit sections must be taken apart and reassembled as the marine riser conduit is raised and lowered from the ocean's floor.
Even more important, however, the structure of the conduit and the forces exerted thereon created problems not present in less stringent applications. To prevent damage to and/or loss of valuable equipment, the marine riser conduit section couplings must resist tension and bending loads created therein. In particular, they must be designed to resist "stress concentrations" which are defined as stresses greater than two times the average stress present in the linear riser conduit sections. Since the average stress present in the linear riser conduit sections may be as high as 25,000 pounds per square inch, it can be seen that great care must be taken to assure uniform tensioning of the connectors in the coupling sections.
In the copending patent application Ser. No. 783,636, now U.S. Pat. No. 4,183,562, entitled "Marine Riser Conduit Section Coupling Means" by Bruce J. Watkins and A. Michael Regan, assigned to the common assignee of this application, a coupling for such marine riser conduits is described incorporating a design whereby an optimum curve is employed transferring from a portion adapted for mating with the riser casing to a horizontally disposed flange having the mating surface disposed thereon such that stresses created within the coupling are optimally transferred. In the coupling described in said application, the flanges are connected together by a plurality of radially equally spaced bolts passing through bores in the flange of the coupling upper member threaded into a threaded bore in the flange of the coupling lower member. In particular, in the example as shown in the application there are twelve bolts connecting the upper flange to the lower flange. Because of the critical nature of the fastenings of the two members together, the aforementioned technique employed in engines and pumps whereby the threaded connections are tightened by torque measuring apparatus, cannot be employed to achieve equal compression from each bolt. Variations in threads and the smoothness of mating surfaces rotated relative to one another as well as the presence of friction-producing agents within the threads of the mating parts make torque measurement an inaccurate means of determining the compressive force being created by the connecting bolts. Thus, the standard bolt tightening technique is to grip the bolts individually and apply an extending force thereto as the bolts are snuggly tightened down against the upper flange. Upon release of the external extending force, the restorative forces of the material of the bolt will cause it to contract thereby exerting a compression force against the upper flange equal to the extension force applied thereto. Such a method is time consuming and still prone to inaccuracies inasmuch as all the bolts cannot be simultaneously extended by a common extending force. Thus, in most such tightening operations either by the extension method or the torque wrench method it is common practice to partially tighten the bolts in steps according a preset pattern whereby the members are drawn into equal compressive loading throughout.
Wherefore, it is the objective of the present invention to provide method and apparatus for replacing the bolts employed in such prior art apparatus by fasteners which can be extended simultaneously by a common internal extending force to allow a one-step fastening and unfastening of two members requiring such equalized compressive holding together thereof.